A phase locked loop circuit is used to produce a signal synchronized in phase with a periodically varying reference signal such as a pulse signal fixed in phase. A type of such a phase locked loop circuit has response characteristics variable in response to a response control signal.
A phase locked loop circuit of this type is used where clock pulse components are to be extracted from signals having synchronizing and data sections such as the signals read out from a flexible memory disc or a compact disc. In such an application of a phase locked loop circuit, the response characteristics of the phase locked loop circuit should be controlled so that the circuit is enabled to respond faster to the response control signal for earlier phase lock for the synchronizing sections of the signals. For the data sections of the signals, it is desirable that the phase locked loop circuit be responsive to the reference control signal at retarded timings so that the circuit is enabled to follow only the fluctuations in the wow flutters which may be caused in the rotary system. On the other hand, in extracting horizontal synchronizing pulses from a composite video signal, the phase locked operation is held in abeyance when the equalizer and vertical synchronizing pulses are appearing during vertical retrace-erase-time intervals.
The present invention relates to improvements in a phase locked loop circuit having such capabilites.